1. Field of the Invention
The invention as disclosed in the specification relates to the structure of thin-film transistors (referred to as "TFTs" hereinafter in the specification). The invention also relates to a method for manufacturing the same.
2. Description of the Prior Art
Prior known TFTs are manufactured using a silicon thin-film as formed on a glass substrate or quartz substrate.
Most TFTs that are currently practiced and implemented are those which employ a non-crystalline silicon film (amorphous silicon film) as an active layer thereof.
The amorphous silicon film may be relatively easily fabricated by use of plasma chemical vapor deposition (CVD) techniques.
It has been considered that the technological trend of liquid crystal display (LCD) devices of the active matrix type in near future is to further develop the "system on panel" structure, in which several types of circuits are integrated together on a single glass substrate or quartz substrate, which circuits may include active-matrix circuitry and circuitry for driving the same as well as circuits handling both video image information and a variety of kinds of information items.
To attain such structure required, the currently available TFTs using one or more amorphous silicon films are encountered with a problem: the characteristics thereof remain too low.
Those TFTs using such amorphous silicon films remain low in characteristic, which in turn results in limited applicability--the TFTs are merely applicable to certain active matrix circuits of the active-matrix LCD devices.
More practically, in the TFTs using amorphous silicon films, the mobility is 1 cm.sup.2 /Vs or less. Yet further, only N-channel type ones are implementable for practical use; P-channel type ones remain too low in characteristic so that these cannot be reduced to practice.
Note here that the mobility of metal oxide semiconductor (MOS) transistors using single-crystalline silicon wafers is typically greater than or equal to 1000 cm.sup.2 /Vs.
One approach as partly practiced today to overcome this problem is the use of specific TFTs using a crystalline silicon film.
One method for obtaining the crystalline silicon film is to thermally crystallize an amorphous silicon film upon heat application thereto.
For example, plasma CVD techniques or low-pressure CVD techniques are employable to form an amorphous silicon film, which is then heated at temperatures of 800.degree. C. to 1000.degree. C. for several hours thus obtaining a crystalline silicon film with polycrystalline state.
This method has been called the "high-temperature" process due to the fact that high temperatures are utilized which are needed for fabrication of standard integrated circuits (ICs).
A TFT with the resultant crystalline silicon film thus obtained by the aforesaid method is such that an N-channel type one has its mobility of approximately 100 cm.sup.2 /Vs whereas a P-channel type one has the mobility of 60 cm.sup.2 /Vs or more or less.
With the characteristics of such degrees, it is possible to form complementary MOS (CMOS) circuits as required to constitute integrated circuits. In addition, it may also be possible to employ such TFTs to configure those circuits that have conventionally been arranged by prior known ICs using single-crystal silicon wafers, although these are not sufficient to fully meet the intended characteristics.
However, in order to fabricate the TFT using the crystalline silicon film, it should be required that a specific substrate with increased thermal resistivity (this is limited to quartz only in the current situation) be employed, which would result in an increase in production cost (due to the cost penalty of such quartz substrate).
An alternative approach as currently studied is to make use of a glass substrate of low cost while adequately rearranging the crystallization method per se.
This approach is called the "low-temperature" process due to the fact that fabrication is done through specific processes which require thermal processing at selected temperatures that give no affection to the heat durability of a glass substrate used.
One typical approach incorporating the above concept is a technique for crystallization of an amorphous silicon film at a heat-up temperature that is carefully controlled permitting the glass substrate to thermally withstand.
By way of example, an amorphous silicon film is formed on a glass substrate; then, the resulting structure is heated up at a temperature of 600.degree. C. for 48 hours to thereby obtain a crystalline silicon film.
Unfortunately, a TFT employing such resultant crystalline silicon film will fail to exhibit any satisfactory characteristics.
Another problem is that production costs are not so lower than expected because of an increase in heat-up time duration.
Still another approach to the low-temperature process is a technique for irradiation of laser light to thereby alter or transform in nature an amorphous silicon film into the intended crystalline silicon film.
This approach is advantageous in that a glass substrate used remains almost free from heat application.
It is possible for those TFTs obtainable by this technique (called the "laser process") to attain superior characteristics which may correspond to the characteristics of TFTs as obtained by high-temperature processes.
It has been found that the TFTs as obtained by the prescribed low-temperature process yet remain too low in characteristic to achieve the system-on-panel architecture required.
The technologies required here may include:
(1) Low-temperature process; and PA1 (2) Attainability of even higher characteristics than those of TFTs obtained by laser processes. PA1 (1) channel region; and PA1 (2) interface between the channel region and its neighboring region as well as "nearby" regions thereof.
As the technique for satisfying the requirement items, the applicants of this patent application has developed one specific scheme of crystallization which introduces a minute amount of metal element of an amorphous silicon film for later effectuation of thermal processing to thereby perform crystallization. This technique has been disclosed in Published Unexamined Japanese Patent Application No. 7-321337.
Those TFTs employing such crystalline silicon films obtained by this method offer considerably high performance. However, some of the crystalline silicon films obtained by this method can contain therein residual metal elements as have been used for crystallization processes, which residual elements can badly affect the characteristics of the TFTs.
Practically, with regard to the items of the reliability and the uniformity of characteristics among device, it has been affirmed that the TFTs remain less as compared to the conventional TFTs of low characteristics.
A Study by the present inventors has revealed the fact that the low reliability of device characteristics and low uniformity of characteristics thereof are originated from affection of such metal elements that continue residing within crystalline silicon films fabricated.